Non-virtual Reality: Why are cardinals red and frogs green?

“Good morning. Baker, Baker, Brown and Gray; how may I direct your call?”

With our two adult daughters long since out of the house, it’s just Deb, me, the chocolate lab and the little gray tabby cat at home now. We sound like a law firm.

Cute. How about Baker, Baker, Green and Blue? Just as good names for senior partners in the practice maybe, but who ever heard of a grass-green dog or a cat with sky-blue fur? That’s just silly.

Why is it silly? Green frogs are green, bluebirds are blue. Why doesn’t mammal fur come in such shades? Why do I need a dye to color the fuzz on top of my head an outrageous cardinal red, but cardinals don’t?

Well, actually they sort of do (see below), but anyway…

Studies on the color of animal feathers, fur and skin have led to more than a few surprises. For instance, the blue jay’s blue feathers are slate-colored, not blue, as is the goldfinch’s seemingly lemon yellow plumage.

Recall that, as a prism shows, sunlight is composed of a variety of types of light — some visible as the colors of the rainbow and others, like ultraviolet and infrared, undetectable by our eyes. “Pigments” are chemicals that absorb some of those forms of light while reflecting others. The most common pigment in plants, for example, is chlorophyll which absorbs all forms of visible light (which are then used in photosynthesis) except green.

Thus, a lush, healthy forest is green because green light is the least usable part of sunlight for plants. (Imagine what your lawn would look like if the structure of chlorophyll allowed it to absorb green light too — a lush, healthy black.)

A blue jay's color results from how the feathers scatter the sunlight that reaches our eyes.(Photo11: Submitted)

Chlorophyll may be the most common plant pigment, but it’s not alone. Over 8,000 plant pigments have been identified, which helps explain the gorgeous diversity of hues seen in the world’s flowers and fruits. Compare that with the mere handful of pigments animals can synthesize on their own. By far the most common of these are the several forms of melanin.

In both vertebrates and invertebrates, eu-melanins impart a black or brown color while pheo-melanins result in sandy pink to reddish tones. As I age, my hair follicles are producing less black eumelanin, turning my hair gray. Blondes have a small amount of the brown eumelanin and no other pigments in their hair.

About 2 percent of the human population are redheads possessing relatively high concentrations of pheomelanin and little eumelanin in their hair. But is a redhead’s hair (or a red fox’s fur) truly red — fire engine, chili pepper, cardinal red? Animal-produced pheomelanin isn’t up to the job.

So for a cardinal to get its deep red plumage — or a goldfinch its lemon yellow body or the Baltimore oriole its burnt orange chest — each has to seek out seeds and fruits with the appropriate pigments. And those pigments will almost always belong to the large class of chemicals known as carotenoids, which range in color from beet red to canary yellow.

How important are plant carotenoids in the diet? Given a nutritionally adequate diet, but one from which the carotenoids have been removed, an adult male cardinal will turn an ashy gray after several molts as the eumelanin embedded in his feathers are no longer masked by red pigments. Same thing for a goldfinch’s yellow plumage.

A blue jay's feather is slate gray, but its blue color comes from reflecting specific types of light.(Photo11: Submitted)

Blue is different story. If a normal blue jay feather is viewed under polarized light it appears gray. Take it back into full sunlight and it’s blue again. This isn’t a case of pigmentation — animals don’t make a blue pigment — but an effect of the feather’s structure.

Tiny air sacs covering blue feathers scatter light in such a way that it’s mostly the blue component of sunlight that makes it to our eyes. (In a similar fashion, nitrogen and oxygen gases in the Earth’s atmosphere scatter incoming rays of the sun, creating the blue sky of a cloudless summer day.)

The startling iridescence on the neck of the aptly named ruby-throated hummingbird works in a comparable fashion. Each of a male’s throat feathers is covered with thousands of miniature bowl-shaped depressions. At the bottom of every depression are microscopic stacks of crystalline plates that refract light (coming in at just the right angle) to yield that glimmering red.

If it’s a heavily overcast day, the feathers’ eumelanin pigments will make his neck appear black. But under sunlit conditions, the pigments absorb most of the incoming light allowing the iridescent red to stand out more clearly.

Greens, like blues, are also usually structural colors. There’s no green pigment in the skin of a frog. Rather, sunlight scattered by a deep layer of skin cells reflects blue wavelengths back through an over-layer of yellow pigmented skin.

Blue + yellow = green.

Ken Baker is a retired professor of biology and environmental studies from Heidelberg University. If you have a natural history topic you would like Dr. Baker to consider for an upcoming column, please email your idea to fre-newsdesk@gannett.com.